The present invention relates to a hydraulic control for an automatic transmission and, more particularly, to a shift timing control for an automatic transmission.
A previously proposed hydraulic controls for an automatic transmission is disclosed, for example, in JP-A 58-156757, (equivalent to U.S. Pat. No. 4,563,918). An automatic transmission as described in this Japanese document is of the four forward speeds type, and includes three clutches, two brakes and one one-way clutch for shift control. A low clutch is engaged in first, second, and third gears, whereas a high clutch is engaged in third and fourth gears. A band brake is engaged in second and fourth gears. Accordingly, when the low clutch and the band brake are engaged, second gear is established; when the low clutch and the high clutch are engaged, third gear is established; and, when the high clutch and the band brake are engaged, fourth gear is established. When shifting from fourth gear to third gear, the band brake is released, whereas the low clutch is engaged. In this case, shift timing is adjusted by regulating the buildup of release pressure for releasing the band brake and that of low clutch pressure for engaging the low clutch. When shifting from fourth gear to second gear, the high clutch is released, whereas the low clutch is engaged. In this case, shift timing is adjusted by regulating the fall of high clutch pressure, and the buildup of low clutch pressure. The low clutch includes a single piston, and it is constructed to operated by hydraulic pressure working on the piston on one side thereof, and return by a spring. Accordingly, as working pressure, there is only hydraulic pressure for engaging the low clutch. The band brake includes an apply chamber for engagement and a release chamber for release, and undergoes two working pressures: an apply pressure and a release pressure which are supplied to the two chambers. Accordingly, four working pressures participate with a shift between any two gears of the second, third, and fourth gears. However, only two hydraulic pressures are variable to achieve a shift between any two gears of these gears. Accordingly, at a 4-3 shift and a 4-2 shift, timing of the two hydraulic pressures may be adjusted.
Even with a similar planetary gear train as described in the above Japanese document, however, when a servo unit for controlling the band brake includes three hydraulic chambers, i.e., a second gear apply chamber, a third gear release chamber, and a fourth gear apply chamber, and that the low clutch includes two hydraulic chambers, i.e., a low clutch apply chamber and a low clutch release chamber, six working pressures participate with a shift between any two gears of the second, third, and fourth gears: high clutch release pressure, low clutch apply pressure, low clutch release pressure, band brake second gear apply pressure, band brake third gear release pressure, and band brake fourth gear apply pressure. Particularly, low clutch release pressure changes with band brake fourth gear apply pressure upon a 4-3 shift, and with high clutch pressure upon a 4-2 shift. Accordingly, low clutch release pressure should be adjusted to change not only in a predetermined relationship with band brake fourth gear apply pressure, but in a predetermined relationship with high clutch pressure. The prior art has not proposed means for solving such a problem.
It is, therefore, an object of the present invention to provide a shift timing control for an automatic transmission which allows an appropriate change of low clutch release pressure in a 4-3 shift and a 4-2 shift.